So just how big is the current Big Freeze - and how bad is it for Britain's wildlife? I'm just too young to recall the worst winter of the 20th century - 1962-63, but I can recall spells of freezing weather in 1979, 1982, and again in the mid-1980s, all of which had a major effect on our wild creatures. Since then we've had a pretty unbroken run of very mild winters, at least until last year.

So far, the current freeze is ranking about fourth in the 'league table' of the worst winters since the Second World War - about the same as 1979, when I recall a major influx of wildfowl from the Netherlands to the gravel pits and reservoirs around my London home.

That winter the freezing conditions lasted the whole of January, and were followed by a thaw, then another freeze-up in February. Three years later, 1982 saw even colder weather, with a new record low of minus 27 degrees at Braemar in Scotland.

But as I recall, much of the western part of Britain was ice-free during these winters, allowing birds to find sanctuary there - unlike the current conditions, where even Wales, the south-west and Ireland are snowbound. So the big question facing the forecasters - and Britain's wildlife - is: how long will this freeze last?

If it thaws in a week or so, and February is mild, then the current winter will rank with 1979, and the long-term effect on our wildlife will not be very severe. If, however, the blocking high continues to bring freezing weather from the east, and even more snow and ice, then we may see the kind of carnage our grandparents witnessed in the winters of 1940, 1947 and of course 1963.

What happened to Britain's wildlife in the 20th century's harshest winters?

1963 really was the Big One - the freezing weather lasted from Boxing Day until the first week of March, and snow fell somewhere in Britain every day for two months. The effect on our birdlife was devastating: as ornithologist James Fisher (the David Attenborough of his day) observed at the end of the winter: "it seems likely that at least half the wild birds living in the country before last Christmas are now dead".

Many birds simply fled the country, while millions of birds from the east took their place. But even so, large parts of eastern Britain were virtually bird-free for several weeks. All sorts of unusual visitors were found in gardens - including water rail, woodcock and grey partridge!

But for many birds, the only option was death: Britain's kingfisher population fell by 85-90%, as did that of the wren, goldcrest and Dartford warbler - a scarce heathland species that was all but wiped out, with just a dozen pairs remaining.

The winter of 1947 was in many places even worse than 1963 - producing an enormous death toll for ground-feeding birds such as the lapwing, skylark and redwing. That was because the cold spell came late that year, in February, when the birds' fat reserves were at their lowest, and they didn't have the strength to get away from the snow and ice.

If we do get a thaw in February this year, this winter will be most like 1940: short and sharp. Again, sedentary species such as the wren, goldcrest and treecreeper suffered badly. In Worcestershire, a flock of seven woodpigeons were found stuck to tree-boughs by ice. And in Dumfries, a flock of starlings took cover under a parked car during a snowstorm. Unfortunately the snow became so deep the car could not be moved, and the birds perished.

When we recall these earlier hard winters it's worth remembering an important difference: very few people fed garden birds in those days - especially during the two big freezes during and after the Second World War when food was rationed. Our national habit of feeding birds today will undoubtedly make a huge difference to the fate of some of our favourite species.